Industrial robots have become an indispensable part of modern manufacturing. Whether transferring semiconductor wafers from one process chamber to another in a cleanroom or cutting and welding steel on the floor of an automobile manufacturing plant, robots perform many manufacturing tasks tirelessly, in hostile environments, and with high precision and repeatability.
In many robotic manufacturing applications, it is cost-effective to utilize a relatively generic robot arm to accomplish a variety of tasks. For example, in an automotive manufacturing application, a robot arm may be utilized to cut, grind, or otherwise shape metal parts during one phase of production, and perform a variety of welding tasks in another. Different welding tool geometries may be advantageously mated to a particular robot arm to perform welding tasks at different locations or in different orientations.
In these applications, a tool changer is used to mate different tools to the robot. One half of the tool changer, called the master unit, is permanently affixed to a robot arm. The other half, called the tool unit, is affixed to each tool that the robot may utilize. When the robot arm positions the master unit adjacent a tool unit connected to a desired tool, a coupler is actuated that mechanically locks the master and tool units together, thus affixing the tool to the end of the robot arm. Operation of the robot arm, as well as many peripheral devices such as the master unit of a tool changer, is controlled by software executing on a robot controller. The robot controller may receive inputs from a variety of sensors (force, pressure, temperature, optical, etc.).
Robotic tools may require utilities, such as electrical current, air pressure, hydraulic fluid, cooling water, electronic or optical data signals, and the like, for operation. Connections to these utilities may be unwieldy, or even unsafe, in operation. Additionally, if two or more tools require the same utilities, a dedicated connection to each tool would be duplicative. Accordingly, one important function of a robotic tool changer is to provide utility-passing modules. Such modules may be attached to standardized locations on the master and tool units of the robotic tool changer. The modules include mating terminals, valve connections, electrical connectors, and the like, making the utilities available to the selected tool when it is coupled to the robot arm. Many tool changers include one or more standard-sized “ledges” about their periphery, to which various utility-passing modules may be attached, as required. Tool changers and utility-passing modules are well known in the robotics arts, and are commercially available, such as from the assignee, ATI Industrial Automation of Apex, N.C.
When not in use, robotic tools are stored in a special rack, or tool holder, within the operative range of the robotic arm. Robot arm controller software “remembers” where each tool is, and each tool is returned to precisely the same position in its tool holder prior to the tool changer decoupling. Similarly, the robot arm controller software “knows” precisely where the next desired tool is stored, and it positions the master unit of the tool changer (on the robot arm) adjacent the tool unit (on the desired tool), then actuates the tool changer to couple the tool to the robot arm.
Many tool changers include location features, such as conical pins protruding from the master unit and corresponding tapered holes in the tool unit, to ensure the master and tool units assume the proper relative geometry as they are coupled together. However, ideally these features should not be relied on for alignment, as they require the robot arm to shift or rotate the tool in its tool holder as the master and tool units of the tool changer align. Rather, the robot arm should ideally adjust its spatial orientation and its position to precisely align with the tool residing in the tool holder, so that the master and tool units of the tool changer are already in alignment as they are brought together for coupling.
When initially setting up a control program, such as for a production run, the robot controller must be “taught” where each tool resides in storage. This information includes not only the tool's location, but its spatial orientation, which the robot arm must match to ensure a proper coupling by the tool changer.
One known method of training a robot controller regarding tool location is to attach plates, called teaching aids, to the master and tool units of the tool changer. For example, one of the teaching aid plates presents a mechanical interface to the master unit that mimics the tool unit, and includes the tapered holes to receive the conical location pins protruding from the master unit. When the teaching aid plate is abutted to the master unit, the tool changer coupling mechanism is actuated to couple the teaching aid to the master unit, as if it were a tool unit. Similarly, a teaching aid plate, e.g., having conical location pins to mimic the master unit, is placed over the tool unit, and may be held in place by magnets or the like. Alignment marks are etched into or painted onto the sides of the teaching aids.
To train the robot controller software of a tool location, a technician positions the robot arm to an approximate position near the tool, such as by using a joystick or other user interface to the robot controller. As the robot arm is moved forward to bring the two teaching plates together, the position of the robot arm is manually adjusted, in small increments, to bring the alignment marks on the two teaching aids together. The alignment marks on all sides of the teaching aids should be precisely aligned as the robot arm is moved to abut the teaching aid plates together. The spatial orientation and position of the robot arm at this point—after adjusting the z-axis position to account for the thickness of the teaching aid plates—is then stored as the location to attach and detach the tool. This process is repeated for each tool the robot arm will utilize during the production run.
The teaching aid plate method of training a robot arm regarding the position of robotic tools has several disadvantages. The accuracy of alignment between master and tool units is limited by human visual perception, which is limited and highly inconsistent from person to person and from day to day. In cases of misalignment, it may be difficult to ascertain whether the alignment is simply in the x-y plane, or whether the robot arm is rotated, canted, or otherwise not in the same spatial orientation as the tool. In the case of large tool changers designed to handle very heavy tools, the master and tool units may include multiple mechanical coupling mechanisms. Providing teaching aid plates for each of these, and attempting to simultaneously align multiple teaching aid plates, is impractical. Finally, even when satisfactory alignment is achieved, a correction in the z-axis position is required, to account for the thickness of the teaching aid plates.
The Background section of this document is provided to place embodiments of the present invention in technological and operational context, to assist those of skill in the art in understanding their scope and utility. Unless explicitly identified as such, no statement herein is admitted to be prior art merely by its inclusion in the Background section.